The long-term objectives of this project are a better understanding of the effect of conductive hearing loss on the binaural hearing of adults and children. The first aim of the present project will investigate factors that contribute to poor masking-level difference performance in adult conductive lesion patients, before and after middle ear surgery. This area will include 1) Investigation of the time-course of MLD performance, measuring before surgery, soon after surgery, and then at regular intervals over a four year period, in order to examine possible recovery from sound deprivation. 2) Investigation of potential causes of poor binaural hearing (besides deprivation) before and after middle ear surgery, including acoustic cross-over, and interear phase mismatch. 3) Investigation of the potential for better predicting binaural hearing after middle ear surgery. 4) Investigation of an asymmetry effect, where the effect of conductive asymmetry appears to be much greater than simulated asymmetries in normal-hearing subjects. 5) Investigation of a second asymmetry effect, where MLD performance in asymmetric cases appears to be poorer for signals presented to the "bad" ear than signals presented to the "good" ear. Investigation of these issues will aid our understanding of binaural hearing in conductive hearing loss, potential recovery from deficits in binaural hearing after cessation of sound deprivation, and potential for better predicting post-surgical binaural advantage. The MLD will be measured using both standard and insert earphones, in order to assess possible importance of crossover, and using a range of simulated inter-ear time delays, in order to offset possible delays caused by middle ear pathology or surgical prosthesis. The two ears will be stimulated at both equal SL and equal SPL. The second major area of the project will investigate the effect of otitis media no the binaural hearing of children. This research will investigate the hypothesis that auditory deprivation in early childhood may result in poor binaural hearing, as measured by the MLD. One experiment will examine the MLD in children aged 5, 7, and 9 years old, who have normal hearing but histories of OM before age 5 years. A second experiment will examine the MLD longitudinally (over four years) in 4-5 year old subjects still experiencing OM on entry into study. This second study will provide a detailed description of the time course of changes in binaural hearing during and following auditory deprivation, which will be directly comparable to the adult longitudinal data (see above). Response to cues of interaural time and level will be measured independently. The factors of asymmetry of disease and time after resolution of hearing loss will be evaluated. In both project areas the psychophysical method will be three- alternative forced-choice testing with sound presentation over earphones. All phases of the project will include age-matched control subjects whose results will further our understanding of normal binaural hearing. Data will be analyzed using analysis of variance and correlation procedures.